Electronic motor control apparatus with separate null indicating device



Aprl 10, 1951 A. E. GEALT 2,548,0l4

ELECTRONIC MOTOR CONTROL APPARATUS WITH SEPARATE NULL INDICATING DEVICE Filed June e, 1949 /SO IO2 FIG.2

E INVENTOR. ARTHUR E. GEALT ATTORN EY.

Patented Apr. 10, 1951 ELECTRON IC MOTOR CONTROL APPARATUS WITH SEPARATE NULL IN DICATIN G DE- VICE Arthur E. Gealt, Philadelphia, Pa., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporatien of Delaware Application June 16, 1949, Serial No. 99,408

12 Claims.

The present invention relates to motor control apparatus of known type including a motor having a motor energizing circuit means connected to the output circuits of. two electronic motor drive valves, each of which includes an anode, a cathode, and a control electrode, and an electronic amplifier connected to the control electrodes of said valves and Operating to make one of said valves more or less conductive than the other in response to the departure in one direction or the other from a normal or zero value of a control signal amplified by said amplifier.

A general object of the invention is to provide new and improved means for indicating the departure and direction of departure of said control signal from is normal or zero value under certain conditions in which the operation of the motor cannot be observed or in which other op erating conditions requireormake desirable the effective disconnection of the motor energizing circuit means from the output circuits of the motor drive valves, or involving the dynamic testing of the motor to determine its torque and speed characteristics with varying degrees of departure of the control signal from its normal or zero value.

The invention may be used with especial advantage in connection with the recalibration or standardizatien of potentiametric instruments of the self-balancing type for measuring and utilizing for control and analogous purposes minute direct current potentials such, for example, as those derived from thermocouples Or from photovoltaic or hydrogen ion cells. .It is cus- I tomary in measuring and controlling instruments of this type to provide a battery or dry cell for energizing the measuring circuit to produce a voltage of known magnitude to which the thermocouple or other voltage under measurement is Compared. Because of the tendency of the voltage output of batteries to decrease or change with age, use and fiuctuations in the ambient temperature to which they are subjected, it is necessary or desirable from the standpoint of maintainng measuring and controlling accuracy periodically to check the voltage of the battery and to eect a compensating adjustment of the measuring circuit for such changes in battery voltage as may have occurred so as to maintain the voltage of known magnitude ata substantially constant value. such a compensating adjustment is known in the art as a recalibrating or standardizing adjustment and ordinarily is accomplished by disconnectng the thermocouple from the measuring circuit and by comparing the known voltag to the vo1tage of a standard cell from which little or no current is d-awn during the compensating operation and which provides a substantially constant voltage throughout its useful life.

The need for such compensating provisions is especially apparent in potentiometric measuring and -controlling instruments of the so-called narrow span type. By a narrow span potentiometric instrument is meant one which is so designed that the range of measurement is small and highly suppressed, for example, from 900 F. to 1000 F., as Compared with a range of 0 F. to 1000 F. or greater in potentiometric instruments of ordinary span.

In many control applications for which such narrow span potentiometric instruments are especially applicable, it is imperative for the recalibrating or standardizing adjustments of the potentiometric measuring circuit to be, made without efiecting momentary, false adjustments of the final control element which tend to occur in the prior art instruments as a direct result of the recalihrating Operations. The difculty with the prior art measuring and controlling instruments in this connection is that the valve controlling the supply of fuel to an oil-fired high temperature furnace, for example, may be given rapid closin and opening adjustments during the standardizing operation and as a direct result of that operation. In such event, the resulting squirting of the fuel oil into the hot furnace may cause a big, explosive puff. The furnaoe fiame may be extinguished by this puff or, where safety devices responsive to the presence or absence of the fiame are provided, the safety device may cause the fuel valve to close. Such closure of the fuel valve is undesirable because of the interruption of the process and also because of the attendant difficulties due to the necessity of reopening the fuel valve and resettng the safety device.

A specific object of the present invention is to provide an interlock between the recalibrating meehanism of the potentiometric measuring circuit and the motor energizing circuit means of the potentiometrie rebalancing means for preventing operation of the rebalancing motor durthe recalibrating operation So that the potentiometric measuring apparatus is made inoperative to change the position of the final control element during recalibrating Operations. A more specific object of the invention is' to provide simple and eficient means in association with such apparatus for indicating the changes 'and the direction of the changes which have occurred in the battery voltage subsequent to the last recalibrating adjustment. In accordance with the present invention, such indication is utilized as indicating the necessity for a 'ecalibratin adjustment and the extent of the recalibrating adjustment required.

The present invention may also b-e used to advantage in connection with the dynamic testing of motors such as the rebalancing motor of the potentiometric measuring apparatus hereinbefore described. Thus, the indicating means provided in association with the motor drive valves may be employed to provide a measure of the torque and speed characteristics oi the motor with control signals of different magnitude and phase app-lied to the control electrodes of the motor drive valves. In making such dynamic tests of the motor, it is -customary to load the output shaft of the motor or otherwise interfere with its free rotation and in making such measurements, therefore, it is impossible to employ the rotation of the motor shaft as a measure of the character of the control signal.

The various features of novelty which characterize this invention are pointed out with particularity in the claims annexed to and forming a part of this speeification. For a better understanding of the invention, however, its advantages and specific cbjects obtained with its use, reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. 1 illustrates a motor control arrangement embodying the principles of the present invention;

Fig. 2 is a circuit dia-gram illustrating the use of the motor control arrangement of Fig. 1 in a rehalancing potentiometric measuring and controlling apparatus; and

Fig. 3 is a side elevation of one practical form of the standardizing switch and calibrating rheoi stat of the potentiometric measuring apparatus of Fig. 2.

In Fig. l, a transformer i Supplies energizing current to a motor energizing circuit means which may well be, and is shown as heing, of the general type disclosed and claimed in the Wills Patent &423540 granted July 8, 1.947. As diagrammatically shown, the transformer i comp rises a primary winding 2 having its terminals connected to alternating current supply conductors L' and i L adapted to supply alternating current of conventonal frequency and voltage: for example, 60 cycles per second and 115 volts. The transforrner also includes two secondary windings, respectively designated by the numerals 3 and The secondary winding has one end terminal connected to the anode of a triode 5 and has its other end terminal connected to the ane-de of a second triode 6. The triodes 5 and may well be the two triodes included in a twin tube of the commercially available 'TNT type. The cathode of the triode valve is connected through an adjustable resistor l and a fixed resistor 8 to a grounded conductor 9. The cathode of the triode is connected through fixed resistors iii and H to the grounded conductor 9. The latter conductor is connected through the control winding i of a motor M to the miri-point of the secondary winding 3 through a center tap is. A condenser i i in parallei with the winding !2 cooperates with the latter to form a parallel resonant circuit.

The power winding !5 of the motor M is connected across the supply conductors L' and L in series with a condenser lt to form a series resonant circuit. The motor M is of conventional rotating field type, and the control winding !2 and the power winding !5 cooperate to create a motor field which rotates in one direction or in the opposite direction, depending upon the phase of the control signal impressed on the control electrodes of the valves 5 and 6.

The control electrodes of the valves 5 and G are connected to one another and are coupled through a condenser H to a terminal 8 which, together with a terminal !9, forms the pair of input terminals to which the control signal is adapted to be applied. The terminal !9 is connected directly to the grounded conductor 9 and is connected. by a fixed resistor 20 to the control electrodes of the valves 5 and 6.

In accordance with the present invention, the fixed resistors 8 and ii are each shunted by an individual condenser 2! and 22, respectively, and

a meter 23 and a resistor 24 are connected in series with each other between the terminals of the condensers 2! and 22 which are remote from the grounded conductor 9. The meter 23 is of known type and, for example, may comprise a milliammeter.

In the intended operation of the apparatus shown in Fig. 1, the resistors l and 8 in the cathode circuit of the valve 5 and the resistors i@ and ll in the cathode circuit of the valve 6 serve as biasing resistors to provide cathode bias voltage for the associated 'valves 5 and E, respectively. The condenser 'Zi in shunt to the resistor 8 and the condenser 22 in shunt to the resistor il serve to smooth out any uctuations or ripple in the voltage drop which may be produced across the resistors 8 and ii as oathode current flows through said resistors.

suitable and suitably related values of the circuit Components shown in Fig. 1 are as follows:

Resistor l-750 ohms Resistor 8-50 ohms Resistor lil-500 ohms Resistor l i-50 ohms Resistor 24-10,000 ohms Condenser 21-20 microfarads Condenser 22-20 microfarads It is noted that the resistors 8 and H are each of the same value: namely, 50 ohms. Consequently, when the current in the cathode circuit of the triode 5 is equal in magnitude to that of the triode 6, the voltage drops produced across the resistors 8 and li will be identical. That condition of identity will be indicated by a zero or undefiected reading of the milliammeter 23. In other words, when the voltage drops across the resistors 8 and are equal, the potential of the junction of resistor 8 and the condenser ZI will be the same as the potential of the junction of the resistor H and the condenser 22 and no current will then flow through the milliammeter 23. This condition of zero current flow through the milliammeter 23 can occur only for the operation of the valves 5 and 6 in which both valves are equally conductive during the altcrnate half cycles of the alternating Voltage provided by the supply conductors L' and L in which they are permitted to conduct. Zero current flow through the milliammeter 23 thus indicates that zero control signal is applied to the terminals !8 and !9.

When a controlsignal of one phase with respect to the supply conductors L' andL is applied to the control electrodes of the valves 5 and &one valve or the other, depending upon the phase of the control signal, will be made more conductive than the other by an amount depending upon the ampiitude* of the control signal. As a consequence' of valves 5 and 6 being difierently con'- ductive, the Vo'ltage drop produced across resistor 8 by theflow of current in the cathode circuit of valve 5' will be made dierent from the voltage drop produced across the resistor H by the flow of current in the cathode circuit of the valve 6. For example, when Valve 5' is more conductive than valve 8, the Voltage drop produced across resistor 8 will be great-er than that produced across resistor l i and Vice Versa.

As a result of the inequality in the voltage drops across the resistors 8 and ll, a current will be established through the milliammeter 23 in one z.

direction or the other accordingly as the Voltage drop across the resistor 8 is larger or smaller than the voltage drop across' the re istor H. The direction of deection of the' indicating pointer of the milliammeter 23, therefore, Will provide an indication of which valve 5 or G is more oonductive, and, consequently, an indication of the phase of the control signal applied to the terminals i& and |9-. The extent of de-flection or departure of the indicating pointer of the milliammeter 23 from its Zero or normal position will be indicative of the extent to which one valve 5 r 5 is made more conductive than the other, and, aecordingly, is indicative of the amplitude of the control signai applied to the input terminals !c'i and iii.

With this arrangement, therefore, the defieo tion of the milliammeter 23 may be utilized to indicate whether a control signal is applied to the terminals !8 and le and to indicate the phase and amplitude of-the control signed that is applied to those terminals.

For the purpose of compensating for rence in the conductivities of the valves &i and which may exist for the condition of zero control signal applied to their control electrodes, the resistor l' is made adjustable. Thus, with zero control signal appiied to the terminals !3 and l, the resistor 'l is adjusted until the milliammeter 23 indicates the condition of zero current fiow through it which exists only when the voltage drop across the resistor 8 is exactly the same as that produced across resistor ll.

As is shown in Fig. l, the control winding i?. of the motor M is connected in a circuit which is common to both cathode circuits of the valves 5 and S. Thus, When zero control signal is applied to the input terminals sa and i and the milliammeter 3 indicates a zero reading, the frequencies of the Voltage and current in the control Winding !2 are double the supply voltage frequency. In other words, a` puise of current ll then fiow through the motor winding !2 during' each half of the supply Voltage wave,iwhich pulses are of substantially equal magnitude. For this condition of operation, the rotcr of the motor M is not energized for rotation in either direction, and accordingly, the motor rotor remains stationary.

When a control signal is applied to the input terminals !8 and 19', the current pulses delivered to the motor Winding 52 during one half cycle of the supply voltage will be larger or smaller than the current pulses delivered to that winding during the other half cycle according to the phase of the control signal. In consequence of this difierence'in.. the current' pulses during the: opposite half cycles in the motor'windng !2, the current flow in the local circuit including the motor winding !2 and its associated condenser will include an alternating current component of the same frequency as that of the supply voltage. That alternating current conponent or" Voltagc is operative to establish in the magnetic circuit of the moter M a magnetic field which reacts With the magnetic field produced the current fiow in the motor winding 5 to establish a magnetic field in the motor which rotates in one direction or the other, depending upon the phase of the said alternating voltage Component in the winding !2 with respect to the supply voltage.

The amplitude of the alternating current component of voltage so produced in the motor wind ing !2, and which is effective to effect motor rotation, is Variable over an Operating range in ao cordance with the ampiitude of the control sig nal applied to the input terminal !3 and t. As the ampiitude of the control signal is in"- creased, the defiection of the indioating' pointer of the milliammeter 23 is correspondingly increased, thereby to provide a measure of the am plitude of the controLsigna-l.

It is apparent, therefore, that the motor control arrangement shown in Fig. l may be uti lized in making dynamic tests on the motor M and, for example, may be utilized in determining the torque and speed characteristics of the motor with difierent control signals applied to the input terminals !8 and !9.

In Figs. 2 and 3, I have illustrated another use of the motor control arrangement shown in 1 and in particular, the use thereof in a rehalancing potentiometric measuring apparatus. The potentiometric rebalancing apparatus shown in Figs. 2,* and 3 may be utilized for.. measuring, indicating, recording and controliing .ie Value oi a variable condition: .for example, the temperature withina furnace 25. Furnace 25 may be heated by a burner 25 controlled by a valve 2? operated by an air operated motor 28.

A thermocouple T responsive to the teinperature within the furnace 25 operates in conj unction with a potentiometric network, generaliy desig nated by the reference numeral Zi?, and a slidewire assembly, generally designated 35', to form a selfbalancing potentiometer instrument. Upon a change in temperature within the furnace 25,- an unbalanced direct current Voltage is produced in one direction or the other depending upon the sense of unbalance of the i potentiometric network 29. The unbalanoed direct current voltage soderived is impressed upon the input cirouit of an electronic device 3 i. The motor drive arrange ment including the electronic Valves 5 and shown in Fig. 1 comprises the output portion of the electronic device 3!. The input portion of the electronic device 3! includes a Vi'orator or equivalent device, an input transformcr, and a suitable Voltage amplifier, the output circuit of which is connected to the input circuit of the electronic val /e; 5 and 6. The vibrator, input transformoi' and voltage amplfier of the electronic device 31 prof'- erably are of the type disclosed the afor mentioned Wills Patent 2423549.

The unbalanced direct current voltage derived fromthe potentiometric network 29is translated by the vibrator into a pulsating current of one polarity or the opposite polarity depending upon the .sense of unbalancc oi: the potentiometric net- Work. This pulsating current is converted into an alternating current by the input transformer,

which alternating current is amplified by the voltage amplifier. The amplified output of the voltage amplifier is applied to the input terminals !8 and !9 of the motor drive Valve arrangement as shown in Fig. l and operates through that arrangement to control the energization of the reversible motor M for rotation in'o ne direction or the other. The motor M operates a driving mechanism, generally designated 32, and rotates in one direction or the other, depending upon the direction of unbalance of the potentiometric network 29. The drive mechanism 32 operates the slidewire assembly 30 to rebalance the potentiometric network and also operates indicating and recording means for indicating and recording the temperature existing within the furnace 25. The

drive mechanism 32 also is arranged to operate a controller, generally designated at 33, which may take the form of an air-operated controller such as is illustrated in Patent 2,l25,081, granted July 26, 1938, to C. B. Moore. This controller 33 is operatively connected to the valve 21 and adjusts the latter as required to maintain the temperature within the furnace 25 at the desired, normal Value.

As shown, the potentiometric network 29 includes three resistors 34, 35 and 36 connected in series( These resistors are preferably formed of manganin wire having a substantially Zero temperature coefiicient of resistance and are employed for calibrating purposes. A battery 31, which may be a dry cell, is connected in series with a dual vernier rheostat, indicated at 38, in a branch in the potentiometric network 29 which is parallel to that including the resistors 34, 35 and 36. The rheostat 38, as shown, includes movable resistors 39 and 53 and electrically-connected stationary contacts ll and 42, respectively associated with the resistors 39 and 43. The rheostat 33 is arranged for manual adjustment by means 'of the manipulation of a knob 43. The knob &3 is arranged for direct manual connection to the movable resistor 40 and is arranged for mechanical connection to the movable resistor 39 through a lost motion mechanism including a pin :te so that upon initial movement of the knoh 43, the resistor 43 is first moved and then the resistor 33 is moved, thereby providing a fine and a coarse adjustment of the rheostat 38. The Construction and operation of the rheostat 38 is explained in greater detail in connection with Fig. 3.

' The potentiometric network 29 includes a second branch which is connected in parallel With that including the resistors 34, 35 and 36. That second branch includes series connected resistors 45 and 43. Resistor 45 is preferably made of copper or other material having a positive temperature coefcient of resistance. Resistor 56 may be formed of manganin having a substantially zero temperature coefiicient of resistance. The resistor 35 operates to compensate for changes in ambient temperature at the cold junctions of the thermocouple 'I' in a manner well known in the art. The resistor 33 is utilized primarily for standardizing or recalibrating purposes and its Value is so selected that the voltage drop across it is equal to the voltage produced by a standard cell indicated by the numeral 41.

The slidewire assembly 33 includes a cable drum 48 which is rotated by the drive mechanism 32 through a cable 49. The cable 39 may be made of wire, violin string, rope or cord. The cable drum 48 operates through an adjustable connection to move a slidable contact 53 along a gage contacts 74, ?5, ?6, 7:2' and ?8.

slidewire resistance 51. The resistance 5! comprises resistance wire suitably space wound on an insulated core 52. The resistance 5! and core 52 preferably are formed of material, such as manganin, having a zero temperature coefficient of resistance. The slidable contact 55 also engages a collector bar in the form of aresistance wire 53 space wound on a core 54 and completely electrically conductive with respect thereto throughout theirlengths. The resistance wire 53 and core 53 are iormed of the same material,

such as manganin wire, as the resistance 5! and nected to opposite ends of the resistor 35 in the' potentiometric network 29, whereby the slidewire resistor 5l, the resistor 53 and the resistor 35 are all connected in parallel.

A two-position switch, generally designated by the reference numeral 30, is provided for accomplishing two functions: specifically, for adjusting the potentiometric network 29 into its normal Operating condition and for adjusting the potentiometric network into its standardizing or recalibrating condition. As shown, the switch 60 includes switch arms ti, 62, 63 and 34 which are electrically insulated from each other and all of which are movable by a common Operating member 55. The 'switch arms 6! and 52 carry contacts 61 and 63, respectively. The switch arm 62 carries an additional contact 59 and the switch arm 33 carries a contact w. The switch arm 64 carries a contact 'I i.

In the normal Operating or running condition, the switch arms El through 65 are in the position shown in Figs. 2 and 3 wherein the contacts 5? and 68 respectively engage relatively stationary contacts 12 and ?3 and the contact Gi' also engages a movable contact M.

In the standardizing position of the switch Si), the contacts (il, 63, 69, 'lil and li respectively en- When the switch 63 is moved to its standardizing position, the contacts 61 and 58 disengage from their respectively associated contacts 72 and ?3.

One form of mechanism which may be employed for adjusting the switch from its normal running position to its standardit ing position is illustrated in Fg. 3. As shown in Fig. 3, that mechanism is adjustably associated with the vernier rheostat 38 and the knob 43 for simultaneously adjusting the switch into its standardizing position and for bringing the knob 43 into operative relation with the vernier rheostat 38 for adjusting the latter.

The vernier rheostat 33, as hereinbefore noted, includes a fine adjustment consisting of a fine resistance 40 and a coarse adjustment consisting of a coarse resistance 33. The resistors ai) and 33 are respectively wound on associated discs '53 and 83, desirably made of insulating material, and mounted on a shaft ai. The disc lil is rigid with the shaft se, and, therefore, rotates directly with that shaft'. The disc 33 is connected to the shaft 81 through a lost motion connection including the pin 44 and a slot in the disc 83 so that the dlsc 85 effectively is rotated by the disc '39 when the pin 44 engages the extreme ends of said slot. Thus, the fine resistance i is adiusted directly by the shaft Si and the coarse resistance 39 is adjusted by the fine resistance through the lost motion connection including the pin M. The resistors 4& and 39 are engaged by the clectrically connected Contacts 42 and 4! which serve the purpose of shunting out more or less of the resistances 49 and ca from the potentiometric branch including the battery 37 as the shaft si is rota-red. By their frictional engagement with the resistors 48 and 33, the Contacts 42 and 4! serve also to prevent accidental rotation of the resistere and 39.

As shown, the rheostat 33, the knob the shaft Si and the switch ti) are all. mcunted within an instrument oasing desi nated at Merr ted on the instrument casing 52, adjacent and abi 4 the switch is a leaf sp ng 33 which is adapted for cngagei ent and movement by an adjustahle screw 84 which, in turn, is

mcunted on a lever 8., pivoted at 86. 'In order to move the switch iii to its standardizing position, the lever is moved a counter-clockwise direction to bring the leaf spring 83 into engagement with the common erating member of the switch to. such counter-cloclwise motion of the lever is accomplished by engagement between a pin si and cam surface 38 that is formed on a shait 33 on one end of which the knob 43 is mounted. shaft 89 is journalled for axial and rotative rnovement in the instrument casing 82 in suitable bearings. Shaft 86 may be rotatcd an ir^ *i axialiy by means of knob 33, s i idar ing 'the instrument, the knob 43 is positioned the left as see in Fig. 3 to move common operating member or the switch se downwardly, thereby to effect adjustrnent of the switch ea into its standardizing position. Continued inotion of the knob 43 to the leit operatcs to brin a clutch plate &ti into engagenient with a second clutch plate si, which latter clutch piate rigidly mounted on the end of shaft ?ii of the rheost 33. 'After the clutch plates 33 and si have e.. gaged, the knob 43 may be rotated to adjust the amount of the resistors and which are connected in the potenticmetric branch in series with the battery 3?.

r l operates a pinion 92 which drives a gear The gear 93 carries a gear 4 and a cahle drum The cable 49 which drives the slidewire contact 50 of the slidewire assembly 3 is fastened to the cable drum 95 and pass-es ov r a 98 carried by a lever 9? pivot -d at downwardly by a spring 59. T e *able also pass.:n

L over pulleys me and iii and passes over the cahle drum 48. The gear 92 operates a gear !92 which in turn cperates a pen ar for recording the temperature variations withi. the furnace 25 on a chart 34 and also positions arm for controlling the air operated device 33. The chart [84 is arranged to he rotated a constant speed by the chart drive motor me through suitable mechanism not shown.

The arm operates a fiapper rnechanisn iii? carried by the body portion EES of the air operated device 33. The devioe 33 is equipped w th dials [09 and a for adjusting the throtthng range and the rate of reset of the control device. Air under pressure is supplied through a pipe H! to a pneumatic relay nechanism 2, and the air pressure in a pipe li3, controlled by the control device 33, regulates a pilot valve contained in the relay mechanisn 2 to produce a pressure in the pipe ii@ in accordance with the temperature within the fu**- nace 25. The prcssure in the pipe il i is conducted by a pipe ii to the pneumatic motor 23 which operates the valve 2?, and the pressure in the pipe H l is also connected through a 'pipe ll& to the control device 33 in order to provide desired foilow-up and reset operations in the control device 33. i

Upon an increase in temperature within the furnace 25, motor M is operated in the counterclockwise direction to drive the gear 33 in a clockwise direction. Rotation of the gear sa in the clookwise direction causes the contact 50 of the slidewire assembly 35 to rotate in a clockwise direction to rebalance the potentiometric network '29. Clockwise rotation oi"- the gear 93 also causes the gear sector ::32 and the pen m to rotate in a counter-clockwise direction to record on the chart iii the increase in temperature which has occurred within the furnace 25. Counter-clockwise rotation of the gear 32 also operates through the arin and fiapper assembly ie? to actuate the control device 33 to position the relay mechanisin !12 to decrease the pressure transmitted to the pneumatic motor 28 of the valve 21. This moves the valve 27 towards its closed position to decrease the supply of fel to the furnace 25 for the purpose of reducing the temperature of the furnace to the desired, normal value. The decrease in pressure transmitted to the valve motor 28 is also transmitted to the control device 33 to provide a follow-up action for causing the valve 2? to be positoned in accordance with temperature value within the furnace. This decrease in pressure transmitted to the control device 33 `operates through the'reset mechanism thereof to position additionally the Valve 2? towards a closed position if the temperature increase within the furnace extends over a substantial period of time. i

Upon a decrease in temperature within the furnace 25, the opposite action takes place. Specifically, the slidewire contact 5@ is inoved in a counter-clockwise direction, the pen arm !83 is moved in a clockwise direction'to a lower temperature value along the chart Hi t, and the control device 33 is actuated to cause a proportionate opening of the valve 2? to increase the supply of fuel to the 'fur-hace 2%, restoring the furn'ace temperature to the desired Value.

In accordance with the present invention, the need for adjustment of the rheostat 38 by the knob is indicated by a deection of the indicating element of the miliiamneter 23 from its normal undefiected position when the switch ti) is in the standardizing position. The direction of defiection from normal of the milliammeter indicating element indicates whether the portion of the resistere 39 and iii included in the potentiometric branch with the bat e'y 31 should be increased or decreased. With this arrangement, therefore, the standardizing or recalibrating adjustment which is required may quickly and easily be rrade by manipu ation of. the knob Moreover, in making such adjustment, it is not necessary to observe the operation of the potentiornetric rehalancing motor, nor the mechanism operated thereby, as in the prior art as disclosed in the afcrenentioned Wills patent, for example. This feature of my invention is of especial importance in certain control applications, as will become apparent from the following explanation, in that the potentiometric rebalancin motor M may temporarily be made inoperative during the standardizing adjustments, whereby any tendency for false adjustments of the fuel control valve 21 to be made during the standardizing Operations is eliminated. This tendency has existed in the prior art apparatus, as disclosed in the aforementioned Wills patent, for example, and has created diiiiculties making that appaartus unsuitable for certain control applications.

In explanation of this difliculty with the prior art apparatus, and on the assumption that a substantial change occurs in the'voltage of the battery 31 and that the rebalancing motor M is operatively connected to the output circuit of the electronic device 3! during the standardizing adjustments, the adjustment of the switch 60 to its standardizing position may cause rotation of the motor M of a magnitude sufficient to effect rapid movement of the 'final control valve 27 to its fully opened or closed positions. Thereafter, upon adjustment of switch te to its normal running position, the final control valve 21 will rapidly be returned to a position near that occupied prior to the standardizing adjustment. Such rapid and wide adjustments of the valve 21 permit rapid changes in the flow of fuel through the nozzle 26 into the hot interior of the urnace 25 and are effective to cause the fuel to be ignited with more or less explosive violence. In the case in which the furnace 25 is rotected by a fiame failure device or other type of safety device, such an explosion might cause the furnace 25 to be shut down with attendant difiiculties in restarting.

Such explosions and attendant dimculties are prevented in accordance With the present invention by rendering the potentiometric rebalancing motor M temporarily inoperative during the standardizing adjustments so that no false adjustments in the position of the fuel valve 21 can be eifected as a result of the standardizing operation. As 'shown in Fig. 2, the mechanism by which the motor M is made inoperative during the standardizing adjustments includes the standardizing switch contacts 'il and 18 which engage in the standardizing position of the switch 60 to Shunt the power winding of the motor and thus deenergize the motor for rotation. Deenergization of the motor M for rotation in this manner does not interfere with the operation of the electronic device 31, nor with the operation of the milliammeter instrument 23 for indicating the state of balance of the standardizing portion of the potentiometric network 29. Accordingly, the standardizing adjustment needed may be made and the apparatus restored to its normal running condition without producing any false adjustments of the fuel valve 21.

While, in accordance with the provisions of the statutes, I have illustrated and described the best form of the invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention as set forth in the appended claims, and that in some cases certain features of the invention may sometimes be used to ad.- vantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters 'Patent is:

l. In a motor control apparatus of the type comprising a pair of motor drive valves, each of which has an anode, a cathode, and a control grid, and an output circuit including the anode and the cathode of the valve, the output circuits of the two valves including separate current supply connections to the anodes of the two valves and a common current supply connection to the cathodes of the two valves, a motor, motor energizing circuit means connected to the output circuits of said valves for operation of said motor in one direction or in the opposite direction when the anode current of one of said valves is respectively larger or smaller than the anode current of the other valve, and means for applying a control. signal to the control grids of the two valves to make one valve more or less conductive than the other when the value of the control signal differs in one sense or in the opposite sense from the normal value thereof, theimprovement comprising a pair of series connected resistors between each of the cathodes of the two valves and said common current supply connection, and a Voltage responsive device connected between the adjacent ends of the resistors of one pair and the adjacent ends of the resistors of the other par.

2. A motor control apparatus as specified in claim 1, including a separate capacitor connected in parallel to each of said resistors adjacent said common current supply connection.

3. A motor control apparatus as specified in claim 1, in which the resistors of each pair adjacent said common current supply connection have approximately the same resistance, and in which at least one of the other two resistors is adjustable to permit Variation in its resistance.

4. A motor control apparatus as specified in claim 1, in which the resistors of each pair adjacent said common current supply connection have approximately the same resistance, in which at least one of the other two resistors is adjustable to permit variation in its resistance to compensate for such differences in the conductivities of said valves as may exist with a zero control signal applied to the control grids thereof, and including a separate capacitor connected in parallel to each of said. resistors adjacent said common current supply connection.

5. In a motor control apparatus of the type comprising a pair of motor drive valves, each of which has an anode, a cathode, and a control grid, and an output circuit'including the anode and the cathode of the valve, the output circuits of the two valves including'separate current supply connections to the anodes of the two valves and a common current supply connection to the cathodes of the two valves, a motor having a power wincling and a control winding, a current supply connection to said power winding including a capacitor connected in series with said power Winding, means connecting said motor control winding to the output circuits of said valves for operation of said motor in one direction or in the opposite direction when the anode current of one of said valves is respectively larger or smaller than the anode current of the other valve, and means for applying a control signal to the control grids of the two vaives to make one valve more or less oonductive than the other When the value of the control signal differs in one sense or in the opposite sense from the normal value thereof, the improvement comprising a. pair of series connected resistors between each of the cathodes of the two valves and said common current supply connection, a voltage responsive device connected between the adjacent ends of the 13 resistors of one pair and the adjacent ends of the resistors of the other pair to indicate the state of conduction of said valves, and means independent of the state of conduction of said valves to deenergize said motor for rotation.

6. A motor control apparatus as specified in claim 5, in which sai last mentioned means comprises a low resistance connection including a. 'switch therein to Shunt said motor power winding upon closure of said switch.

7. In a motor control apparatus of the type comprising a pair of motor drive valves, each of which has an anode, a cathode, and a control grid, and an output circuit including the anode and the cathode of the valve, the output circuits of the two valves including separate current supply connections to the anodes of the two valves and a common current supply connection to the cathodes of the two valves, a motor having a power winding and a control winding, a current supply connection to said power winding including a capacitor connected in series with said power winding, means connecting said control winding to th output circuits of said valves for operation of said motor in one direction or in the opposite direction when the anode current of one of said valves is respectively larger or smaller than the anode current of the other valve, and means 'for applying a control signal to the control grids of the two valves and Operating to make one vaive more or less conductive than the other when the value of the control signal differs in one sense or in the opposite sense from the normal value thereof, the improvement comprising a pair of series connected resistors between each of the cathodes of the two valves and said common current supply connection, the resistors of each pair adjacent said common current supply connection having approximately the same resistance and at least one of the other resistors being adjustable to permit variation in its resistance to compensate for such differences in the conductivities of said valves as may exist with a zero control signal applied to the control grids thereof, a, separate capacitor connected in parallel to each of said resistors adjacent said common current supply connection, a voltage responsive device connected between the adjacent ends of the resistors of one pair and the adjacent ends of the resistors of the other pair to indicate the state of conduction of said valves, and means independent of the state of conduction of said valves to deenergize said motor for rotation.

8. In a self-balancing potentiometer instrument, the combination of, a potentiometer circuit including a pair of terminals on which a unidirectional electromotive force to be measured is adapted to be impressed, a source of known unidirectional electromotive force for producing a uni-directional electromotive force in opposition to the uni-directional electromotive force impressed on said terminals, a slide-wre for Variably opposing the source of known urn-directional electromotive force to the electromotive force impressed on said terminals, an electronic amplifier having an input circuit controlled by the potentiometer circuit and having an output circuit in which a control voltage difiering in one sense or in the opposite sense from a, normal Value thereof is adapted to be produced in accordance with the state of balance of said potentiometer circuit, a pair of motor drive valves, each of which has an anode, a cathode, and a control grid, and an output circuit including the anode 14 and the 'cathode of the valve, the output circuits of the two valves including separate current supply connections to the anodes of the two valves and a common current supply connection to the cathodes of the two valves, a motor for Operating said. slide-wire, motor energizing circuit means connected to the output circuits of said valves for operation of said motor in one direction or in the opposite direction when the anode current of one of said valves is `respectively larger or smaller than the anode 'current of the other valve, a connection from the output circuit of said electronic amplifier to the control Vgrids of the two valves and Operating to make one valve more or less conductive than the other when the Value of the control voltage differs in one 'sense or in the opposite sense from the normal value thereof, a pair of series connected resistors between each of the cathodes of the two valves and said'common' current supply connection, a voltage responsive device connected between .the adjacent ends of the resistors of one pair and the adjacent ends of the resistors of the other pair to indicate the state of conduction of said valves, a standard'celi for producing a standard uni-directional electromotive force, standardiZing-switch means for opposing the source of known electromotive force to the electromotive force produced by the standard cell in lieu of the electromotive force impressed on said terminals to check the known electromotive force, and means operated by said switch means to deenergize said motor for rotation when the standard electromotive force is 013- posed to the known electromotive force.

9. In a, self-balancing potentiometer instrument comprising a potentiometer circuit including a pair of terminals on which a uni-directional electromotive force to be measured is adapted to be impressed, a source of known uni-directional electronotive force for producing a uni-directional electromotive force in opposition to the uni-directional electromotive force impressed on said terminals, a slide-wire for variably opposing the source of known uni-directional electromotive force to the electromotive force impressed on said terminals, an electronic amplier having an input circuit controlled by the potentiometer circuit and having an output circuit in which a control voltage differing in one sense or in the opposite sense from a normal value thereof is adapted to be produced in accordance with the state of balance of said potentiometer circuit, a pair of motor drive valves, each of which has an anode, a cathode, and a control grid, and an output circuit including the anode and the cathode of the valve, the output circuits of the two valves including separate current supply connections to the anodes of the two valves and a common current supply :connection to the cathodes of the two valves, a motor for Operating said slide-wire and having a power winding and a control winding, a current supply connection to said power winding including a capacitor connected in series with said power winding, means connecting said control winding to the output circuits of said valves for operation of said motor in one' direction or in the opposite direction when the anode current of one of said valves is respectively larger or smaller than the anode current of the other valve, and a connection from the output circuit of said electronic amplifier to the control grids of the two valves and Operating to make one Valve more or less conductive than the other when the value of the control voltage difiers in one sense or in the opposite sense from the normal value thereof, the

improvement comprising a pair of series connected resistors between each of the cathodes of the two Valves and said common current supply connection, the resistors of each pair adjacent said common current supply connection having approximately the same resistance and at least one of the other resistors being adjustable to permit variation in its resistance to compensate for such differences in the condu ctivities of said Valves as 'may exist with a zero control signal applied to the control grids thereof, a separate capacitor connected in parallel to each of said resistors adjacent said common current supply connection, a, voltage responsive device connected between the adjacent ends of the resistors of one pair and the adjacent ends of the resistors of the other pair to indicate the state of conduction of said Valves, a standard cell for producing a standard uni-directional electromotive force, standardizing switch means for opposing the electromotive force produced by the source of known electromotve force to the electromotive force produced by the standard cell in lieu of the electromotive force impressed on saidterminals to check the known electromotive force, and means operated by said switch means to deenergize said motor for rotation when the standard electromotive force is opposed to the known electromotive force.

10. A self-balancing potentiometer instrument as specified in claim 9, in which the said last mentioned means comprises a low resistance connection including a switch therein to shunt said motor power winding upon closure of said switch, said switch being adjusted to its closed position when said switch means opposes the standard electromotive force to the source of known electromotive force.

ll. In an electric self -balancing potentiometric' instrument having a circuit for standardizing said potentiometric instrument by connecting it to a standard cell by means of a standardizing switch, an electrc rebalancing motor having a power winding and a control winding, leads adapted to connect said` power winding to a source of electricity, leads adapted to connect said control winding to receive a controlling signal therethrough, a condenser connected in one of said power-winding leads to vary the phase of the electricity conducted therethrough, an electric switch adapted when closed to be connected across said power-winding leads in parallel with said power-winding and thereby to prevent motion of said rebalancing motor, and an operator for simultaneously moving the standardizing switch of the potentiometric instrument to standardizing position and ior closing said power-Winding paralleling switchi 12'. A self-balancing potentiometric instrument as specified in claim 11 including standardizing mechanisrn to recalibrate the potentiometric instrument, and an electric meter connected to said potentiometric instrument to indicate the state of balance of said potentiometric instrument.

ARTHUR E. GEALT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,334,704 Hilierty Nov. 23, 1943 2,446,l53 Belcher July 27, 1948 2,464=,818 Learned Mar. 22, 1949 OTHER REFERENCES "A -Continuous-Control servo System"; by J. T. McNaney, pp. 118-125, Electronics, December 1944. 

